In a typical cellular network, also referred to as a wireless communication system, user equipments communicate via a radio access network to one or more core networks.
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the Radio Access Network (RAN) and possibly one or more Core Networks (CNs) comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro base station, home base station or pico base station, which often is related to transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In the context of this disclosure, a base station as described above will be referred to as a base station or a Radio Base Station (RBS). A user equipment as described above, will in this disclosure be referred to as a user equipment or a UE.
The term downlink (DL) transmission will be used for a transmission from a base station to a user equipment. The term uplink (UL) will be used for a transmission going the opposite direction, i.e. from the user equipment to the base station.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standards for technologies like GSM, HSPA and LTE have been and are currently developed.
In so called Heterogeneous Networks, (HetNets), there is a mix of different cell sizes, realized with different types of node capabilities. Typically, different downlink power levels are used. There are a number of differently sized base stations, which are expected to be deployed with different downlink power levels.
So called macro cells handled by so called macro base stations in the HetNet are sometimes referred to as coverage cells, since they provide coverage over a relatively large area. The macro base stations are sometimes referred to as belonging to a so called macro layer.
Low power base stations, such as micro, pico or femto base stations, as well as relays and repeaters, provide coverage in smaller cells, which may be for example micro, pico and femto cells, in the heterogeneous network. These smaller cells are sometimes referred to as capacity cells. An output power difference between macro sites and micro or pico sites can for example be 10-20 dB, or even more.
In a heterogeneous network deployment with embedded smaller cells, such as micro, pico or femto cells deployed within a large macro cell, the cell borders may be different in downlink and uplink. The reason for this is that the low power base stations as mentioned above typically use a lower output power than the overlaying macro base station, whereas the user equipment power is the same for transmissions to macro base stations and transmissions to low power base stations. In the uplink, it is hence the power capability of the user equipment that is important.
The user equipments served by the low power base stations usually get higher data rates because they are closer to the low power base station than to the macro base station, which results in a better radio link. Also, if fewer users have to share the resources in the macro layer, they can get higher data rates, and there will be more uniform data rates in the system. It is hence important that a significant share of the user equipments are served by the low power base stations in order to offload the macro base stations, since otherwise only the user equipments served by the low power base station reach high data rates.
Within 3GPP Rel. 9, cell selection, i.e. the decision regarding which cell the user equipment should be served by, is based on the power of the respective reference symbols from the choice of cells as measured by the user equipment.
Therefore, the low power base stations have a smaller coverage area, or range, than the macro base stations, due to their relatively low transmit power. Thus, unless there are very strong hotspots, i.e. areas where many user equipments are located, and the low power base stations are placed perfectly in these hotspots, the low power base stations will not be able to absorb a desirable amount of users.
To enable a certain base station, or cell, to serve the desirable amount of user equipments, it may be possible to extend the range of a base station to comprise a so called extended range where user equipments select and are served by the base station, or cell, in question, even if it receives a stronger downlink signal from another base station.
In later releases of LTE, it may for example be possible to extend the range of the small cells by using a cell specific cell selection offset.
By increasing this offset, the low power base station may be favoured in the cell selection procedure, and get to serve more users.
A problem with extended ranges is however that the downlink interference situation may be difficult, since downlink transmissions to user equipments in the extended range may be harmfully interfered by other downlink transmissions. For example, a Physical Downlink Control Channel (PDCCH), which is a downlink channel used for scheduling, transmitted from a low power base station may be severely interfered by downlink transmissions from an overlaying macro base station, particularly when the same frequency, or frequency band, and carrier frequencies are used for the downlink transmissions in the cells served by the low power base station and the cells served by the macro base station.
Functionality for so called time-domain Inter Cell Interference Cancellation (ICIC) comprises a concept called Almost Blank Subframes (ABS), which in this context means that a macro base station will transmit with reduced power and/or activity on some physical resources during certain pre-determined subframes. This way, the low power base station will have some subframes during which interference is low enough for the low power base station to serve its user equipments.
A problem with such solution is, however, that every ABS is a reduction in capacity for the macro base station.